The invention relates to automatic transmission in motor vehicles for carrying out automatic shifting by switching power transmission routes, which may be in turn controlled by the operation of shifting means.
In motor vehicles, automatic transmissions are so constituted as to obtain desired driving characteristics by automatically changing their speed ratios in accordance with the driving conditions. In order to attain this purpose, it is customary to provide a shift map composed of upshifting and downshifting lines for each speed range, the lines being established in relation to the vehicle speed and the engine power output, and to control the automatic transmission to shift the speed ranges according to the shifting map dependent on the changes of traveling states as indicated on the shift map. One example of such shifting control is disclosed in Japanese Laid-Open Patent Publication No. 61-189354.
A typical automatic transmission comprises a plurality of power transmitting means (for example, a plurality of gear trains) by which a plurality of power transmitting routes are constituted, a plurality of shifting means (for example, a plurality of hydraulic clutches) by which one power transmitting route defined by one of the power transmitting means is selected, and a controlling means (for example, a hydraulic control valve) by which operations of the shifting means are controlled. In this transmission, when a point representing a traveling state in a shift map crosses a shiftup line or a shiftdwon line, a shift signal for shifting up or down is sent to the control means whose operation is often controlled by solenoid valves mounted thereon. The hydraulic control valve actuated by the solenoid valves in accordance with the shift signal operates one of the hydraulic clutches to select one power transmitting route determined by selecting one of the gear trains, thereby effectuating shifting up or down.
In such shifting, since the speed reduction ratio (gear ratio) in a pre-shift speed range (a speed range determined by the gear train selected until the shift signal was sent) differs from that in a post-shift speed range (a speed range determined by the newly selected gear train based on the shift signal), it is very important to control shifting so as not to cause jerky or delayed shifting.
For example, if the post-shift range clutch (the shifting means for the post-shift speed range) is engaged too early after the pre-shift range clutch was released during shifting down, the driving power from the running vehicle body is transmitted to the engine, thereby creating engine braking phenomenon. This engine braking power decelerates the vehicle speed to create a jerky shifting. Also, if the post-shift range clutch is engaged too early, both the post-shift and the pre-shift range clutches may engage simultaneously causing jerky shifting, wears of friction members in the clutches being increased and the durability of the clutches (shifting means) being decreased.
Accordingly, shift control methods which can avoid simultaneous engagement of the clutches and moderate shift shocks during shifting to increase the durability of the clutches are disclosed in Japanese Laid-Open Patent Publications 61-109941,61-109942 and 61-109943. In these methods, the pre-shift speed range is shifted to the post-shift speed range through an intermediate speed range in which no engine braking force can be obtained. In other words, the power is not transmitted from the wheel to the engine in the intermediate speed range. Moreover, in case of kick-down shifting in these methods, since the engine speed can be increased rapidly at the intermediate speed range, the time from the start of shifting until the rotational synchronization of input and output members of the post-shift range clutch is shortened, and therefore the overall shifting time can be shortened.
As stated above, the shift via the intermediate speed range without engine brake can reduce chances of simultaneous engagement of both the clutches and moderate shifting shocks. However, if the time set for the intermediate speed range is too short, it is not effective. On the other hand, if the time is too long, the overall shifting time may be enlarged to give embarrassed shifting feeling to the driver or to create engine racing in case of kick-down shifting because the accelerator pedal is depressed. Therefore it is necessary to properly set the time for the intermediate speed range. However, even if this time is set by a timer to a specified value, it is still difficult to obtain a suitable shift control since the time from the transmission of the shift signal until the release of the pre-shift range clutch may well vary from one clutch to another or may vary when the oil temperature changes.